Control devices for controlling the position of a marine seismic streamer

ABSTRACT

A control device or “bird” for controlling the position of a marine seismic streamer is provided with an elongate, partly flexible body which is designed to be electrically and mechanically connected in series with a streamer. In its preferred form, the bird has two opposed wings which are independently controllable in order to control the streamer&#39;s lateral position as well as its depth.

This application is a continuation of Ser. No. 10/704,182, filed Nov. 7,2003 (now U.S. Pat. No. 7,822,552), from which Applicant claims priorityunder 35 U.S.C. §120. Ser. No. 10/704,182 was a divisional of Ser. No.09/893,234, filed Jun. 26, 2001 (now U.S. Pat. No. 6,671,223), which wasa continuation of Ser. No. 09/284,030, filed Apr. 6, 1999 (abandoned),which was a 35 U.S.C. §371 national stage filing from Patent CooperationTreaty application number PCT/GB97/03507, filed Dec. 19, 1997, which inturn claimed priority from Great Britain patent application number9626442.9, filed Dec. 20, 1996, from which Applicant has claimed foreignpriority under 35 U.S.C. §119.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to control devices for controlling the positionof a marine seismic streamer.

2. Description of the Prior Art

A marine seismic streamer is an elongate cable-like structure, typicallyup to several thousand meters long, which contains arrays of hydrophonesand associated electronic equipment along its length, and which is usedin marine seismic surveying. In order to perform a 3D marine seismicsurvey, a plurality of such streamers are towed at about 5 knots behinda seismic survey vessel, which also tows one or more seismic sources,typically air guns. Acoustic signals produced by the seismic sources aredirected down through the water into the earth beneath, where they arereflected from the various strata. The reflected signals are received bythe hydrophones, and then digitised and processed to build up arepresentation of the earth strata in the area being surveyed.

The streamers are typically towed at a constant depth of about tenmeters, in order to facilitate the removal of undesired “ghost”reflections from the surface of the water. To keep the streamers at thisconstant depth, control devices known as “birds”, attached to eachstreamer at intervals of 200 to 300 meters, are used.

Current designs of birds are battery-powered, and comprise a relativelyheavy body which is suspended beneath the streamer, and which has a pairof laterally projecting wings (hence the name “bird”), one on each side.The combination of streamer and birds is arranged to be neutrallybuoyant, and the angle of attack of both wings is adjusted in unisonfrom time to time to control the depth of the streamer.

Birds in accordance with these current designs suffer from a number ofdisadvantages. Because they are battery-powered, the batteries can runout before the survey is completed, necessitating either retrieval ofthe streamer for battery replacement, or deployment of a work boat toreplace the battery in the water. The former operation is very timeconsuming, while the latter can be hazardous. Further, because the birdshang beneath the streamer, they produce considerable noise as they aretowed through the water, which noise interferes with the reflectedsignals detected by the hydrophones in the streamers. The hanging of thebirds from the streamers also means that the birds need to be detachedeach time the streamer is retrieved and re-attached each time it isre-deployed, which is again rather time consuming.

During the seismic survey, the streamers are intended to remainstraight, parallel to each other and equally spaced. However, afterdeploying the streamers, it is typically necessary for the vessel tocruise in a straight line for at least three streamer lengths before thestreamer distribution approximates to this ideal arrangement and surveycan begin. This increases the time taken to carry out the survey, andtherefore increases the cost of the survey. But because of sea currents,the streamers frequently fail to accurately follow the path of theseismic survey vessel, sometimes deviating from this path by an angle,known as the feathering angle, of up to 10°. This can adversely affectthe coverage of the survey, frequently requiring that certain parts ofthe survey be repeated. In really bad circumstances, the streamers canactually become entangled, which though rare, causes great damage andconsiderable financial loss. Current designs of birds can do nothing toalleviate any of these lateral streamer positioning problems.

It is therefore an object of the present invention to provide novelstreamer control devices which alleviate at least some of thedisadvantages of the current designs, and/or which possess morefunctionality than the current designs.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a control devicefor controlling the position of a marine seismic streamer, the devicecomprising a body mechanically connected in series between two adjacentsections of the streamer, sensor means in the body for determining itsangular position in a plane perpendicular to the longitudinal axis ofthe streamer, two opposed control surfaces projecting outwardly from thebody, each control surface being rotatable about an axis which in useextends transversely of the streamer, and control means responsive tocontrol signals and the sensor means for independently adjusting therespective angular positions of said two control surfaces so as tocontrol the lateral position of the streamer as well as its depth.

In a preferred embodiment of the invention, for use with a multi-sectionstreamer which includes an electric power line, the control means is atleast partly electrical and arranged in use to receive electric powerfrom said electric power line.

When the streamer also includes a control line, the control means ispreferably arranged in use to receive control signals from the controlline.

The control means preferably includes at least one electrical motor, andmay also include means for sensing the respective angular positions ofthe two control surfaces.

Conveniently, said two control surfaces rotate about a common axis.

Advantageously, each of the two control surfaces comprises a respectivewing-like member which is swept back with respect to the direction oftow of the streamer.

Preferably, said control surfaces are releasably secured to the body,which may be adapted to be non-rotatably coupled to the streamer.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings, of which:

FIG. 1 is a somewhat schematic representation of a preferred embodimentof a streamer control device in accordance with the present invention;

FIG. 2 is a simple schematic of a control system forming part of thestreamer control device of FIG. 1; and

FIGS. 3 to 5 illustrate the operation of the streamer control device ofFIG. 1.

The streamer control device, or “bird”, of FIG. 1 is indicated generallyat 10, and comprises an elongate streamlined body 12 adapted to bemechanically and electrically connected in series in a multi-sectionmarine seismic streamer 14 of the kind which is towed by a seismicsurvey vessel and which is used in, in conjunction with a seismic sourcealso towed by the vessel, to conduct seismic surveys, as brieflydescribed hereinbefore. To permit such connection, each end of the body12 is provided with a respective mechanical and electrical connector 16,18, these connectors being complementary to, and designed tointerconnect with, streamer and connectors 20, 22 respectively which arenormally used to join together adjacent sections 14 a and 14 b of thestreamer 14.

The bird 10 is provided with two opposed control surfaces, or wings, 24,typically moulded from a fibre-reinforced plastics material, whichproject horizontally outwardly from the body 12 and which areindependently rotatable about a common axis extending substantiallyperpendicularly through the longitudinal axis of the body. Rotation ofthe wings 24 is effected under the control of a control system 26sealingly housed within the body 12. The wings 24 are generally ogival(ie rounded) and swept back with respect to the direction of tow of thestreamer 14 (which direction is indicated by the arrow 28), in order toreduce the possibility of debris becoming hooked on them. To facilitatetheir rapid removal and reattachment, the wings 24 are secured to body12 by a quick-release attachment 30.

As mentioned hereinbefore, the streamer 14 includes hydrophonesdistributed along its length; it also includes control and conversioncircuitry for converting the outputs of the hydrophones into digitaldata signals, longitudinally extending control and data lines forconducting control and data signals to and from the control andconversion circuitry, and electrical power supply lines for supplyingelectrical power from the vessel to the circuitry. All these lines arecoupled together from the streamer section 14 a to the streamer section14 b via respective corresponding lines 32 which extend through the body12 of the bird 10 between the connectors 16, 18. Additionally, thecontrol system 26 is connected to receive control signals and electricpower from respective ones of the lines 32.

The greater part of the length of the body 12 of the bird 10 isflexible, the only rigid parts being the connectors 20, 22, and a shortcentral section which houses the control system 26 and from which thewings 24 project. This central section, which is made of aluminium ortitanium and has holes passing longitudinally therethrough for thepassage of Kevlar or other stress members which bear the longitudinalloads on the body 12, is kept as short as possible, typically around 40cm, so that once the wings 24 have been detached from the body 12, thestreamer 14 can be wound onto and unwound from the large drum used forstoring the streamer, with the body 12 still connected in the streamer.The quick-release attachment 30 permits the removal and attachment ofthe wings 24 to be at least partly automated as the streamer 14 isreeled in and out during the survey.

The reason for providing the elongate flexible parts of the body 12 isto provide enough length for the inclusion of one or more hydrophones orhydrophone groups, should this be necessary to preserve a desireduniform hydrophone spacing along the length of streamer 14. If nohydrophones need to be included, the flexible parts of the body 12 canbe omitted altogether, along with the aforementioned stress members.

The control system 26 is schematically illustrated in FIG. 2, andcomprises a microprocessor-based control circuit 34 having respectiveinputs 35 to 39 to receive control signals representative of desireddepth, actual depth, desired lateral position, actual lateral positionand roll angle of the bird 10 (ie the angular position of the body 12 ina plane perpendicular to the longitudinal axis of the streamer 14). Thedesired depth signal can be either a fixed signal corresponding to theaforementioned 10 meters, or an adjustable signal, while the actualdepth signal is typically produced by a depth sensor 40 mounted in or onthe bird 10. The lateral position signals are typically derived from aposition determining system of the kind described in our U.S. Pat. No.4,992,990 or our International Patent Application No WO 9621163. Theroll angle signal is produced by an inclinometer 42 mounted within thebird 10.

The control circuit 34 has two control outputs 44, 46, connected tocontrol respective electrical stepper motors 48, 50, each of which isdrivingly connected to a respective one of the wings 24. The steppermotors 48, 50 have respective outputs at which they produce signalsrepresentative of the their respective current angular positions (andtherefore of the current angular positions of the wings 24), whichoutputs are connected to respective control inputs 52, 54 of the controlcircuit 34.

In operation, the control circuit 34 receives between its inputs 35 and36 a signal indicative of the difference between the actual and desireddepths of the bird 10, and receives between its inputs 37 and 38 asignal indicative of the difference between the actual and desiredlateral positions of the bird 10. These two difference signals are usedby the control circuit 34 to calculate the roll angle of the bird 10 andthe respective angular positions of the wings 24 which together willproduce the necessary combination of vertical force (upwardly ordownwardly) and lateral force (left or right) required to move the bird10 to the desired depth and lateral position. The control circuit 34then adjusts each of the wings 24 independently by means of the steppermotors 48, 50 so as to start to achieve the calculated bird roll angleand wing angular positions.

FIGS. 3 to 5 illustrate the operation of the bird 10 in the case wherethe streamer 14 is slightly heavy (slightly negative buoyancy), and thebird 10 thus needs to produce lift to maintain the streamer at thedesired depth. This lift is produced by the flow of the water over thewings 24 of the bird 10, resulting from the 5 knot towing speed of thestreamer 14 through the water, and can be changed by changing the angleof attack of the wings with respect to the flow. The magnitude of thelift required for the situation envisaged by FIG. 3 is indicated by thelength of the arrows 60.

If the streamer 14 now needs to be moved laterally to the right (asviewed in FIGS. 3 to 5), the angular position of the left wing 24 of thebird 10 is first adjusted to increase its lift, while the angularposition of the right wing is adjusted to decrease its lift, asrepresented by the length of the arrows 64 in FIG. 4, thus causing thebird 10 to roll clockwise from the position shown in FIG. 3 to theposition shown in FIG. 4. This clockwise roll continues until the bird10 reaches the steady state condition shown in FIG. 5, where it can beseen that the vertical component of the lift produced by the wings 24,indicated by the arrows 66, is equal to the lift represented by thearrows 60 of FIG. 3 required to maintain the streamer 14 at the desireddepth, while the much larger horizontal component, represented by thearrows 68, moves the streamer 14 to the right.

While adjusting the angular positions of the wings 24 of the bird 10,the control circuit 34 continuously receives signals representative ofthe actual angular positions of the wings from the stepper motors 48,50, as well as a signal representative of the actual roll angle of thebird from the inclinometer 42, to enable it to determine when thecalculated wing angular positions and bird roll angle have been reached.And as the aforementioned difference signals at the inputs 35 to 38 ofthe control circuit 34 reduce, the control circuit repeatedlyrecalculates the progressively changing values of the roll angle of thebird 10 and angular positions of the wings 24 required for the bird andstreamer reach the desired depth and lateral position, until the birdand streamer actually reach the desired depth and lateral position.

The body of the bird 10 does not rotate with respect to the streamer 14,and thus twists the streamer as its rolls. The streamer 14 resists thistwisting motion, so acting as a kind of torsion spring which tends toreturn the bird 10 to its normal position (ie with the wings 24extending horizontally). However, this spring returning action, thoughbeneficial is not essential, and the bird 10 can if desired be designedto rotate to a certain extent with respect to the axis of the streamer14.

It will be appreciated that the bird 10 has several important advantageswith respect to prior art birds. Its in-line connection in the streamer14 not only reduces the noise it generates as the streamer is towedthrough the water, but also enables it to derive power and controlsignals via the streamer and so obiviates the need for batteries(although they may still be provided if desired for back-up purposes).But most importantly, it enables the horizontal or lateral position ofthe streamer 14 to be controlled, and not just its depth.

Another significant advantage of the bird 10 is that by virtue of theshortness of the stiff parts of the respective body 12 and easilydetachable wings 24, it does not need to be removed from the streamer 14during winding and unwinding. This saves a considerable amount of timewhen carrying out the seismic survey.

Many modifications can be made to the bird 10. For example, the wings 24can be staggered slightly along the length of the body 12, in order toprovide slightly more room for their respective drive trains.Additionally, the electric motors 48, 50 can be replaced by hydraulicactuators.

What is claimed is:
 1. A device comprising: a housing connectable inline between two sections of a marine seismic streamer; first and secondcontrol surfaces that project laterally outward from the housing andthat are independently rotatable with respect to each other so that, ina first configuration, an angle of inclination of a lateral surface ofeach of the first and second control surfaces is different, and, in asecond configuration, an angle of inclination of a lateral surface ofeach of the first and second control surfaces is configured such that asame lift force is applied on each of the control surfaces; and acontrol system in the housing responsive to control signals thatselectively adjust respective angles of inclination of the lateralsurfaces of the first and second control surfaces to have the firstconfiguration, which causes the control surfaces to jointly operate toproduce a rolling force on the housing, and the second configuration,which causes the control surfaces to jointly operate to produce a forceto adjust lateral position of the streamer without producing a rollingforce on the housing.
 2. The device of claim 1, wherein the housing isconstructed of material selected from aluminum, titanium, andcombinations thereof.
 3. The device of claim 1, further comprising: oneor more sensors in the housing for determining the angular position ofthe housing in a plane approximately perpendicular to the longitudinalaxis of the streamer, wherein the control system is responsive to atleast one of the sensors.
 4. The device of claim 1, wherein the twocontrol surfaces rotate about a common axis.
 5. The device of claim 1,wherein the two control surfaces comprise two opposed wings.
 6. Thedevice of claim 1, wherein the housing is adapted to be nonrotatablycoupled to the streamer.
 7. A device, comprising: a bird connectable toa marine seismic streamer configured to flow through water; the birdincluding first and second wings projecting outward from a housing, eachof the first and second wings including a wing surface that extends froma leading edge of the respective first and second wings to a trailingedge of the respective first and second wings, the first and secondwings being independently rotatable to adjust an angle of attack of thewing surfaces of the first and second wings with respect to a directionof travel of the housing during use; and a control system that controlsrotation of the first and second wings to have a first configurationwhere the wing surfaces of the first and second wings respectively havedifferent angles of attack of with respect to a direction of travel ofthe housing that cause the first and second wings to jointly operate toproduce a rolling force on the bird, and a second configuration where asame lift force is applied to the wing surfaces of the first and secondwings respectively to cause the first and second wings to jointlyoperate to produce a force on the streamer to control lateral positionof the streamer without producing a rolling force on the bird.
 8. Thedevice of claim 7, wherein the wings are releasabley secured to thebird.
 9. The device of claim 7, the bird comprising a body having arigid in line housing and a flexible inline portion.
 10. The device ofclaim 9, wherein the body has a diameter approximately the same as adiameter of the streamer.
 11. The device of claim 7 comprising a controlsystem including sensors for sensing angular position of each of the twowings.
 12. The device of claim 11, wherein the control system furthercomprises: a control circuit in communication with one or more sensorsfor sensing depth, lateral position, and roll angle of the bird.
 13. Thedevice of claim 7, wherein the pair of wings are adapted to rotate abouta common axis.
 14. A method, comprising: connecting a bird to a marineseismic streamer in series therewith between two sections of the marineseismic streamer; and individually controlling angular position of firstand second control surfaces of the bird to adjust lift of the first andsecond control surfaces so as to have a first configuration where thefirst and second control surfaces jointly operate to produce a rollingforce on the bird by adjusting an angular position of the first controlsurface to increase lift of the first control surface, and adjusting anangular position of the second control surface to decrease lift of thefirst control surface, and a second configuration where a same liftforce is applied to the first and second control surfaces such that aforce to control lateral position of the marine seismic streamer isprovided without producing a rolling force on the bird.
 15. The methodof claim 14, comprising: determining position of the bird; comparing thedetermined position of the bird to a desired position of the bird; androtating the control surfaces according to the comparison.
 16. Themethod of claim 15, wherein the control surfaces comprise two opposedwings.
 17. The method of claim 16, wherein the two control surfacesrotate about a common axis.
 18. The method of claim 14, furthercomprising: measuring the roll angle of the bird using an inclinometer;and using the measured roll angle in controlling the bird.
 19. Astreamer control device, comprising: a body adapted to be mechanicallyand electrically connected in series between two sections of amulti-section marine seismic streamer; first and second control surfacesthat project laterally outward from the body and that are independentlyrotatable with respect to each other so that, in a first configuration,an angle of inclination of a lateral surface of each of the first andsecond control surfaces is different, and, in a second configuration, anangle of inclination of a lateral surface of each of the first andsecond control surfaces is configured such that a same lift force isapplied on each of the surfaces; an inclinometer mounted within thedevice to provide a roll angle signal indicating the roll angle of thedevice; a control circuit adapted to selectively adjust respectiveangles of inclination of the lateral surfaces of the first and secondcontrol surfaces based on the roll angle signal to individually adjustlift of each control surface so as to provide the first configuration,which causes the control surfaces to jointly operate to produce arolling force on the body, and the second configuration, which causesthe control surfaces to jointly operate to produce a force to controllateral position of the marine seismic streamer without producing arolling force on the body.
 20. The streamer control device of claim 19,wherein the body further comprises: a flexible, greater part of thebody; and a rigid, lesser part of the body.
 21. The streamer of claim20, wherein the flexible, greater part of the body houses at least oneseismic sensor.
 22. The streamer of claim 20, wherein the rigid, lesserpart of the body houses a control system.
 23. The streamer of claim 19,wherein the body houses at least one seismic sensor.
 24. The streamer ofclaim 19, wherein the body houses said control circuit.
 25. The streamerof claim 19, wherein the control surfaces are defined by vanesdetachable during retrieval without disassembling the streamer.
 26. Thestreamer of claim 19, wherein the control surfaces control the positionof the device in both depth and lateral position.